The present invention relates to resource allocation in communications systems. One advantageous application of the invention relates to allocation of scrambling codes used to distinguish dedicated mobile-to-network connections in a Code Division Multiple Access (CDMA) cellular radio communications system.
A direct sequence spread spectrum (DSSS) system is a wideband system in which the entire frequency bandwidth of the system is available to each user all of the time. A DSSS system employs a spreading signal, i.e., a pseudo-random noise (PN) sequence, that expands or xe2x80x9cspreadsxe2x80x9d the bandwidth of the transmitted signal much more than is required for the transmission of information symbols. The spreading signal is usually called a spreading code or sequence. Different users in a DSSS system may be distinguished using different codes. Such multi-user, DSSS systems are also referred to as direct sequence-code division multiple access (DS-CDMA) systems. Therefore, spreading codes are used to perform two basic functions:
(1) spread the bandwidth of a modulated signal to a larger transmission bandwidth
(2) distinguish among different user signals which are using the same transmission bandwidth in the multiple access scheme.
For purposes of this description, two different terms are used to distinguish between these two spreading code functions. A xe2x80x9cspreading codexe2x80x9d spreads the information symbol stream to higher rate. A xe2x80x9cscrambling codexe2x80x9d scrambles the signal to uniquely identify it with a particular user. Typically, the scrambling operation does not change the rate of or further spread the signal (although it could). Even though separate spreading and scrambling codes are described, they could be combined into a single code that performs both spreading and scrambling (mobile identification) functions.
Orthogonal functions are typically employed to improve the bandwidth efficiency of a spread spectrum system. Each network-to-mobile connection typically uses one member of a set of orthogonal functions representing the set of symbols used for transmission. While there are many different sequences that can be used to generate an orthogonal set of functions, the Walsh-Hadamard sequences are common examples used in CDMA systems. Such an orthogonal set of functions can be used as spreading codes. Thus, on the forward or downlink channel (base station-to-mobile), orthogonal functions are used to minimize multiple access interference among users in the same cell. At the base station transmitter, the input user baseband data, such as digital speech, is multiplied by an orthogonal function with the resulting product then being spread by the base station spreading code and combined with some other spread user data for other mobiles before being transmitted on the RF carrier. At the mobile receiver, after removing the RF carrier, the mobile multiplies the baseband signal by the synchronized scrambling code associated with the base station and then multiplies the resulting product with a synchronized spreading code, e.g., orthogonal function, for that mobile user to suppress the interference due to transmission from the base station to other users.
In the uplink or reverse direction (mobile-to-base station), spreading codes from an orthogonal set need not be used in principle since orthogonality cannot be readily maintained anyway. This is because various mobile stations are located at different distances from the base station, and the mobile-transmitted signals received at the base station are not synchronized. However, it is often desirable to have the possibility of multi-code transmission, i.e., a plurality of spreading codes are used simultaneously. In such a case, it is advantageous that such spreading codes are different members of an orthogonal set, since this would maintain orthogonality among the codes used by a certain mobile-to-network connection. Consequently, orthogonal spreading codes are often used in the uplink direction as well, although it is not strictly needed for reasons of inter-connection orthogonality.
While the downlink scrambling codes are cell-specific, and thus statically allocated in the cell planning process, the uplink scrambling codes are specific to each mobile-base station communication and therefore require an allocation strategy. One strategy is to statically allocate uplink scrambling codes so that each mobile station or mobile station to base station dedicated communication is assigned its own more or less unique uplink scrambling code. Scrambling codes are not shared by other mobiles. The problem with this approach is that there must be some coordination between all manufacturers of mobile stations, and thus, global administration of uplink scrambling codes, which is unnecessary and undesirable.
Therefore, it is desirable to pool the uplink scrambling codes and then dynamically assign them to mobile stations on an xe2x80x9cas neededxe2x80x9d basis. While a dynamic approach is more efficient, there is a possible disadvantage. Since an uplink scrambling code is not known in advance to both the mobile and base stations, both the mobile station and the base station need to be informed of the uplink scrambling code allocated to a particular mobile station for a particular connection. Regardless of whether the mobile station or the base station selects the uplink scrambling code, some sort of control signaling must be used to notify the other station of the selected uplink scrambling code so that the appropriate scrambling and descrambling operations can be performed at both the mobile and base stations. That explicit scrambling code control signaling causes delay and consumes valuable communications resources.
It is an object of the present invention to efficiently allocate communications resources.
It is another object of the invention to dynamically allocate communications resources without delay and without consuming communications resources.
It is a further object of the invention to dynamically allocate communications resources like uplink scrambling codes on an xe2x80x9cas neededxe2x80x9d basis without requiring control signaling between a mobile station and a base station to exchange uplink information explicitly identifying a particular communications resource.
The present invention solves the above-identified problems and meets the above-identified objects (and others) by implicitly allocating communications resources in a dynamic, as-needed fashion. No explicit signaling is employed to exchange information pertaining specifically to the allocated communications resource. Instead, the implicit allocation employs parameter(s) already known to both the network and the mobile. Such unique, already-known parameter(s) is (are) then used to generate or address a communications resource. The parameters might include, for example, information readily acquired from or communicated as a result of a communications between the mobile station and the network over one or more common control channels. Such communications may include a synchronization procedure, a mobile access procedure, a paging procedure, etc. Example parameters may include a system frame number, a system identification, a cell identification, a mobile-associated signature, an access reference corresponding to the mobile station, a time instant when an acknowledgment or other message is transmitted or received, etc.
In a spread spectrum communications system, the channel resource may correspond to scrambling codes used to transmit an information signal from a mobile station to a base station/network. A scrambling code generator in the mobile station determines a scrambling code from one or more parameters available at the mobile and base stations that distinguishes the particular mobile station to base station dedicated communication from other mobile station to base station dedicated communications. The one or more parameters may be available at the mobile and base stations as a result of one or more control signaling (typically initialization) procedures performed to enable communication between the mobile station and base station/network. Advantageously, the one or more parameters are not directly related to scrambling codes used to distinguish among mobile station to base station dedicated communications. This eliminates the need for explicit control signaling exchanged between the mobile and base stations relating to a specifically allocated uplink scrambling code. The information signal is then scrambled using the generated scrambling code and transmitted.
The control signaling procedures may include the mobile station synchronizing with and receiving control channel parameters transmitted over a downlink control channel by the base station and sending a message from the mobile station on a up link, access control channel to the base station including mobile station access parameters. In one example embodiment, the one or more parameters used to generate the scrambling code includes one or more of the downlink control channel parameters and one or more of the mobile station up link access control channel parameters. Also in the example embodiment, the scrambling code generator includes one or more linear feedback type shift registers, the initial contents of at least one shift register being determined using the one or more parameters. A base station transceiver may also use a similar scrambling code generator and the one or more parameters to descramble a received signal of a mobile station using scrambling codes based on the one or more parameters available at the mobile and base stations in order to recover the information signal transmitted by the mobile station.